So I set up my sync software a few days ago after recieving my 4gb microsd card. At that time I was able to transfer about 400mb worth of songs to the device without issue. I have been using the music player without any problems. I also have taken several pictures on the card since. Today I attempted to transfer more media files. However, when I click on the media card in the media manager window, I get the error message: "Activate Mass Storage Mode". This is despite already having done this properly (yes, I went into options > media card and it's already activated). Has anyone else run into this problem? What was the solution?

I have not seen that error, no.

My first recommendation, remove from the USB, pull the battery and reboot the device.

Close DM if open.

Reconnect to the PC via USB and attempt again.

That was my first thought too. I've rebooted the device as well as my computer without resolving the problem.

IF you have Mass Storage Mode enabled, then don't open Desktop Manager, nor Media Manager.

Open My Computer, or your PC file manager and go to the removable drive (sometimes about F:// on your PC. Transfer your files by simple drag and drop.

How do I add media files to my microSD card? - BlackBerryFAQ

I have Mass Storage Enabled and I still can not see my device in my computer. What is going on? I have desktop manager closed.

What model do you have? Your profile here says 7230, which won't have a Mass Storage function.

I have an 8310.

Do you see anything in "My Computer" for a "removable dreve" in the e or f slot? If you already have another device in the e:, f: or g: slot, unplug those or rename those drives to something higher like m: or n:. The Media Card on the BlackBerry likes to in e f or g, as I recall.

ALSO, do you have your USB to the BB plugged in to the PC's main USB slots--not a hub or monitor USB port, direct to the back of the PC--not the front, either.

Yes it is plugged into a main slot.